Recently, a digital still camera generally photographs a correctly exposed still image under low illuminance by controlling a flash device (strobe) which is contained in or externally connectable to the camera. As a control method of this flash device, a flash device control method which emits a known amount of light in advance and determines the light emission amount of photography from the reflected light amount of the emitted light is proposed.
Techniques which continuously photograph and record still images are also known. Examples are a continuous photographing function which continuously captures and records still images at predetermined intervals, and a multi-frame photographing function by which, as shown in FIG. 5, still images continuously captured at predetermined intervals are arranged in a time-series manner, and recorded as one still image.
As disclosed in U.S. Pat. No. 6,067,422, therefore, correctly exposed still images can be continuously photographed and recorded even under low illuminance by controlling the light emission of the flash device in synchronism with this continuous photography of the still images.
FIG. 4 is a block diagram showing a digital still camera which has a still image continuous photographing function and controls a flash device in synchronism with continuous photography. The arrangement of this digital still camera will be explained below.
Light entering the digital still camera is photoelectrically converted by an image sensor 403 through an image forming lens 401 and a stop 402 which controls the incident light amount. The image sensor 403 outputs a signal when driven by an image sensor driver 404 which controls the charge read time and charge storage time. A sample-and-hold circuit 405 samples and holds the output signal from the image sensor 403. A variable gain circuit 406 changes the gain of the output signal from the sample-and-hold circuit 405. A stop controller 407 controls the stop 402.
An image signal processor 408 generates an image signal made up of a luminance signal and color signal from the output signal of the variable gain circuit, generates a photometric value from the result of integration of the luminance signal, and outputs these image signal and photometric value. A read/write memory device 409 temporarily stores the output image signal from the image signal processor 408.
Reference numeral 412 denotes a flash device; and 411, a flash device controller which controls the flash device 412.
The output image signal from the image signal processor 408 is output to a still image recording circuit 414 where the image signal is converted into still image information. A still image recorder 415 records the still image information converted by the still image recording circuit 414.
A switch 413 is used to execute continuous photography. When the switch 413 is operated, still image continuous photography/recording start request information is output to a system controller 410. The switch 403 may also be used as an operating member for starting photography in each of a continuous photography execute mode and single-shot photography execute mode which can be switched. It is also possible to execute single-shot photography or continuous photography in accordance with the operation time of the switch 413.
The system controller 410 controls the system of the digital still camera. In order to give a predetermined level to the output photometric value from the image signal processor 408, the system controller 410 outputs control information to the stop controller 407, image sensor driver 404, and variable gain circuit 406, thereby controlling the aperture of the stop 402, the exposure time, and the gain of the output signal from the image sensor. The system controller 410 also controls continuous photography and the light emission of the flash device.
Continuous photography will be explained below.
When continuous photography start request information is obtained from the switch 413, the system controller 410 outputs to the image processor 408 an instruction to capture a predetermined number of still images at predetermined intervals. In synchronism with this still image capture instruction from the system controller 410, the image signal processor 408 writes the captured image signals in the memory device 409. When the capturing of the predetermined number of still images is complete, the image signal processor 408 sequentially reads out the still image signals stored in the memory device 409, and outputs the readout signals to the still image recording circuit 414. The still image recording circuit 414 records the input still image signals in the still image recorder 415, thereby completing continuous photography.
Control of the flash device will be described below.
When still image photography is to be performed using the flash device 412, the system controller 410 causes the flash device 412 to emit light in a known emission amount before still image photography is performed (this light emission will be referred to as “preemission” hereinafter), and outputs control information of this light emission to the flash device controller 411. The system controller 410 acquires the reflected light amount during this preemission as a photometric value from the image signal processor 408. On the basis of this photometric value during the preemission and a photometric value obtained before the preemission, the system controller 410 determines a light emission amount which gives a predetermined level to a photometric value during light emission (to be referred to as “main emission” hereinafter) which is performed in synchronism with still image photography.
The control of continuous photography and the control of the flash device are independent of each other. Therefore, the flash device can emit light in synchronism with continuous photography. This makes it possible to obtain correctly exposed images even when continuous photography is performed under low illuminance.
The processing performed by the system controller when the flash device is to be controlled in synchronism with continuous photography in the prior art will be described below with reference to a flowchart shown in FIG. 3.
In step S301 of FIG. 3, the system controller determines whether the continuous photography start switch is operated to generate a continuous photography start request. If the request is generated, the flow advances to step S302.
In step S302, the system controller initializes number-of-captured-image information indicating the number of captured still images, and acquires a photometric value before preemission. Then, the system controller determines the light emission amount of preemission in accordance with the brightness of an object, and the flow advances to step S303.
In step S303, the system controller outputs the light emission information of preemission to the flash device controller, and the flash device controller controls the flash device to perform preemission at a predetermined timing on the basis of the indicated light emission information. When the flash device completes the preemission, the flow advances to step. S304.
In step S304, the system controller acquires a photometric value during the preemission from the image signal processor, and the flow advances to step S305.
In step S305, from the photometric value before the preemission and the photometric value during the preemission, the system controller determines the light emission amount of main emission such that a photometric value during main emission has a predetermined level. After that, the flow advances to step S306.
In step S306, the system controller outputs the light emission information of main emission to the flash device controller, and the flash device controller controls the flash device to perform main emission in synchronism with still image capturing. When the flash device completes this main emission, the flow advances to step S307.
In step S307, the system controller writes still images captured by the image signal processor into the memory device, outputs an instruction to perform a still image capturing operation, and updates the number-of-captured-image information. Then, the flow advances to step S308.
In step S308, on the basis of the number-of-captured-image information, the system controller determines whether the number of captured still images has reached a predetermined number of images of continuous photography. If NO in step S308, the flow returns to step S302. If YES in step S308, the flow advances to step S309.
In step S309, the system controller reads out the still images stored in the memory device, and instructs the image signal processor to record the readout images in the still image recorder.
In the conventional flash device control method described above, whenever a still image is captured during continuous photography, the light emission amount of main emission is determined by executing preemission. As shown in FIG. 6, therefore, a time of five fields is necessary from arbitrary main emission to the next main emission. This makes it impossible to shorten the image capturing interval of continuous photography.
To solve this problem, as shown in FIG. 7, it is possible to execute preemission only before capturing of the first still image to determine the light emission amount of main emission in advance, and always perform main emission by the same light emission amount during continuous shooting. By this light emission control, the interval of continuous shooting can be reduced to three fields, shorter than that in the prior art.
This light emission control is more advantageous than the prior art in that the continuous photographing interval can be shortened. However, if the exposure state of a photographing frame changes during continuous photography because, e.g., an object or camera has moved, the exposure state of a photographed image cannot be maintained constant since the light emission amount of main emission is fixed.